<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let year=Number(prompt('输入年份'))
    let month=prompt('输入月份')
    switch (month) {
      case '1':
      case '3':
      case '5':
      case '7':
      case '8':
      case '10':
      case '12':
        alert('有31天')
        break;
      case '4':
      case '6':
      case '9':
      case '11':
        alert('有30天')
        break
      case '2':
        if(year%400===0||year%4===0&&year%100!==0)
        alert('有29天')
        else{
          alert('有28天')
        }
        break;
        default:
          alert('输入错误')
    }
  </script>
</body>
</html>